Re: [stella] Visual questions

 Subject: Re: [stella] Visual questions From: cwilkson@xxxxxxx Date: Tue, 25 Jul 2000 14:16:14 -0400
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In message <4.3.2.7.0.20000725035024.0266d970@xxxxxxxxxxxxxxxxxx>, Glenn Saunde
rs writes:
>Two visual-related questions.
>
>When talking about a byte, the numbering is like this, right?
>
>128 	64 	32 	16 	8 	4 	2 	1

yep.

>What about the bit numbering?  Does it go from left to right or right to left?

Bit numbers, in order, are 7-6-5-4-3-2-1-0.

Basically, the decimal value for each bit position is 2^(bit number).  The
decimal value of the byte is computed by summing 2^(bit number) for all
the bits whose value is "1".

>So when drawing player sprites for use in memory by using graph paper, what
>you see is what you get on the screen if you feed it numbers like the
>above, ala:
>
>X X X X X X X X = 255
>X 0 0 0 0 0 0 0 = 128
>X 0 0 0 0 0 X 0 = 130

Yes, but can you use the numeral "1" instead of "X" to represent true?
An "X" typically means don't know, or don't care.

>Also, how I'm going to explain VDEL is that it has the net effect of

This is good.  You just explained VDEL to me...congrats!  (and thanks! :)

>Also, I've noticed that a lot of source code stores sprite data upside
>down.  Any reason why?  Do they scan memory by decrementing a counter or
>something?

Yes, it's much easier to detect a zero than it is to detect a byte overflow,
because of the zero flag.

-Chris

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